This is a request for support of studies that characterize ion transport across frog skin membranes. Over the past eight years the principal investigator has focused his research activities largely on modelling. From this theoretical work have arisen some well defined and intriguing predictions such as the participation of distinct, but cooperative intracellular electrolyte domaines which determine and regulate global transport characteristics of multilayered epithelial membranes. We now request a modest increase in support to evaluate these models and their predictions in the laboratory. Studies are proposed that will involve three senior investigators at the Medical College of Virginia, the principal investigator, Dr. Donald Mikulecky, pioneer in network thermodynamic modelling and Dr. Charles Watlington, an experienced experimentalist in transport studies in frog skin. A major focus of this proposal is to study Na+, K+ and C1- fluxes, as well as related biophysical events in the nonsteady state (transient) period of drug and hormonal action - beginning with the action of ouabain. This is in contrast with most previous flux studies which have explored "steady state" conditions. This approach will give deeper insight into the dynamics involved in the responses of epithelial cells to drugs and hormones. This research proposal will be done within the context of a very active predoctoral and postdoctoral training program in membrane biology at the Medical College of Virginia. Postdoctoral fellow support for this work will enhance the productivity of this project. In return, this program will continue to be a valuable asset to this area of biomedical education and in particular, the pioneering of the application of network thermodynamic modelling to biological systems. This project also has strong health related implications in the disciplines of gastroenterology and renal medicine, inasmuch as salt and water metabolism and its hormonal control is concerned.